MILD SPOILERS FOR S11 ep 1. I don't recommend reading this if you haven't seen it yet.
When Dean replays every conversation with Cas from the last several years, they always start out the same.
Hello, Dean.
And they always just ended, abruptly, when they had to. It always pissed Dean off, how Cas would just hang up or leave when the conversation was over, without ever saying the conversation was over. He thought Cas knew better than that.
But now he realizes…it was always better than the alternative.
Because Cas told him goodbye and he hung up, right as Dean was able to hear someone else in the background coming closer, as heard the realization in Cas's voice when Cas realized someone was behind him.
It takes everything in him not to cry. And he swears to find Cas as soon as he can, even if he has to scour all of Earth, even if it takes the rest of his life because it cannot end this way. Castiel's last words to him cannot be him saying goodbye, not when Dean still has to apologize for so much. Not when he has to thank Castiel for trying to keep Sammy in control.
And fuck, they last time they had seen each other…it wasn't even long ago, it had been two days ago, but Dean had beaten him and almost killed him, and still, Cas had demanded to know how Dean was, demanded to know if the Mark was gone, and Dean felt the tension ease out of Castiel over the phone when Dean told him yes.
Dean didn't understand how Cas didn't know about the Darkness, but that didn't matter, not when Cas was hurt and MIA; not when he had told Dean goodbye after never saying it in all the time Dean's known him.
He sees Sam's worry and pity, and it angers him. In the far back, the deputy shushes the baby as she starts to fuss. That angers him.
Goodbye. It…it may be sometime before we see each other again.
Dean heard the silent 'if' hanging in the air, heavy and loud. Cas had meant to say if we see each other again, Dean heard it, even if Cas didn't say it.
And that pissed him off.
Dean vowed, he was going to get the deputy and the baby to a safe location, and then he was going to find Cas and disembowel anyone who had laid their slimy hands on his angel, and then they were going to defeat the Darkness together, Team Free Will again.
He would find Cas and Cas would look at him in his Castiel way and he would say Hello Dean, because that's what he did.
And Dean would make sure that Cas never told him goodbye again.